The determination of a non-integer ratio between two signals poses a problem in conventional systems. Currently, it is not possible to precisely compare the phase existing between the edges of the two signals whose ratio it is desired to determine, owing to the fractional part of the ratio.
U.S. Pat. No. 6,094,101, entitled “Direct Digital Frequency Synthesis Enabling Spur Elimination”, proposes the direct determination of the non-integer ratio, by means of a special reference signal, in that it requires a prior frequency synthesis even before seeking to determine the non-integer ratio. This synthesis, however, involves several additional components, and is therefore costly in terms of surface area and in terms of power consumption.
Texas Instruments, in the article “Digital Phase-Domain PLL Frequency Synthesizer” and in U.S. Pat. No. 6,326,851, has devised a method that includes over-sampling signals whose frequency is not fixed. The method also includes the direct calculation of the non-integer ratio from each phase-shift (owing to the fractional part) between the edges of the over-sampled signal and the edges of the signal whose frequency is fixed. This method, however, does not allow the fractional and integer parts of the ratio being determined to be obtained in a simple and low-cost manner in terms of power consumption and surface area. As a result, it is not really compatible with methods for digital processing of the fractional part, with a view for example to refining its value or to eliminating any possible noise on the signal.
This is therefore a need for systems and methods for determining a non-integer ratio between a first signal and a second signal.